When I was in high school, I was 5' 8" and 105 pounds...I was healthy but picky eater...I had always been under a doctor's care since I was so underweight...

You should talk to your freind and see what is going on. She should be seeing someone if she is not. If she is, she should be glad someone is so concernes.

BTW I now weigh between 135 - 140 lbs depending on the time of the month!